The first detection of gravitational waves in 2015 created huge
excitement because it confirmed a long-standing prediction of Albert
Einstein’s general theory of relativity and opened up a completely new
way of observing the universe. Physicists have also been scrutinizing
data from the growing number of gravitational-wave detections for
“echoes” – the existence of which could mean that our understanding of
relativity is incomplete. Physicists in Canada and Iran have found
tentative evidence for such echoes gravitational waves from colliding
black holes, and now say a stronger signal exists in data from colliding
neutron stars.

“So far everyone who has looked for echoes has found them, including the LIGO group.”

—Niayesh Afshordi of the University of Waterloo and the Perimeter Institute for Theoretical Physics.

Many physicists believe that general relativity is incomplete because
it is at odds with quantum mechanics, leading to the information
paradox when considering the extreme gravitational fields generated by
black holes. Relativity tells us that whenever anything, including
light, crosses a black hole’s event horizon the information it contains
is lost to the rest of the universe forever. But quantum mechanics
requires that information can neither be created nor destroyed. This is a
problem given the existence of Hawking radiation, which implies that
black holes can evaporate away to nothing and in the process erase all
of the information that flowed into them.

If gravitational-wave echoes exist, it would suggest that black holes
are not bounded by a classical event horizon but instead by a
quantum-mechanical Planck-scale structure. One such structure put
forward by theorists is the “firewall”, which would destroy any object
passing through it but retain that object’s information and so keep it
outside the black hole. Firewalls, however, are controversial. While
physicists generally agree that quantum mechanics comes into play deep
inside black holes – even though it is impossible to see its effects –
they are largely skeptical about its role outside the event horizon.

Barrier bouncing

Gravitational-wave echoes would be created thanks to the presence of
the Planck-scale structure, or “membrane”, and what is known as the
angular momentum barrier. The latter is a boundary lying around 1.5
times as far as the event horizon (typically around 200 km from the
center of a black hole) that is predicted by relativity and which
partially confines gravitational waves. Any outgoing wave generated
between the event horizon and the barrier would normally bounce off the
barrier and then pass through the horizon, never to be seen again. But
the membrane, lying within a Planck length of the horizon, would instead
reflect the wave back, allowing it to either bounce off the barrier
again or, less likely, pass through the barrier into space.

As a result, the barrier can act like semi-reflective mirror that
releases a small fraction of the gravitational-wave energy into space
after each reflection from the membrane. This would appear as weak
bursts of gravitational radiation – the echoes – separated by a
well-defined time interval that depends only on the black hole’s mass
and rate of spin.

This proposal is based on an idea originally put forward by Vitor
Cardoso of the University of Lisbon in Portugal and colleagues in
February 2016, just a couple of weeks after the LIGO collaboration in
the US had announced the first detection of gravitational waves. Then in
December that year, Niayesh Afshordi of the University of Waterloo and
the Perimeter Institute for Theoretical Physics in Canada and colleagues
said they had evidence to back up the idea, claiming to have found a
2.5σ signal for the echoes in gravitational waves from three pairs of
merging black holes, including that seen in the first detection.

Consistent with noise?

That claim was met by scepticism from nine members of the LIGO
collaboration, who did their own analysis of the data. They included
more background than considered by Afshordi’s team and colleagues, and
found a signal, but with less significance – about 2σ. The result, said
the LIGO team, was “entirely consistent with noise”. They therefore
concluded that the rival analysis did “not provide any observational
evidence for the existence of Planck-scale structure at black hole
horizons”.

Undeterred, Afshordi and his colleague Jahed Abedi of the Sharif
University of Technology in Tehran looked for echoes in data from the
merging neutron stars announced with much fanfare by LIGO and Virgo in
Italy in October 2017. First, they calculated the range of expected echo
frequencies and time delays between merger and echoing – 60-90 Hz and
up to 1 s, respectively (the latter depending on whether the neutron
stars collapsed directly to form a black hole or first produced a very
massive neutron star). They then scanned the data set to find out
whether there were waves matching those criteria. As they reported
recently on the arXiv server, they did indeed find such a signal – at 72
Hz, around 1 s after the merger. What’s more, they found only a few
similar repeating patterns at other times within the data. As such, they
claim, the signal has a significance of 4.2σ.

Cardoso says it is “puzzling” that the neutron-star echoes should
have a higher significance than those from the merging black holes –
given that the latter signal was more intense. He also cautions that the
repeating waves could be a consequence of conventional physics, such as
“radiation from leftovers of the merger”. Nevertheless, he argues that
the prospect of new physics makes such searches worthwhile. “It would be
foolish not to dig deep into this,” he says.

Afshordi admits he was surprised to find such a strong signal in the
neutron star data, and acknowledges that fresh observations from LIGO
and Virgo will be needed to settle the issue. But he argues that the
evidence is building, pointing out that another group, at the University
of Toronto, has seen 3σ evidence for the echoes. “So far everyone who
has looked for echoes has found them, including the LIGO group,” he
maintains. “We have yet to have a group that doesn’t find anything.”